The Great Lakes of North America are the largest expanse of fresh water on the planet. Searching for clues of their formation, our geologists delve deep into an underground salt mine, investigate a fossilized coral reef, climb an Alpine glacier, and dive to the bottom of Lake Superior. They find evidence of an ancient tropical sea, a mighty rift that almost tore the continent in half, and a mile high ice sheet that repeatedly carved its way across North America. And as the lakes settle to their current levels, cascading over Niagara Falls, we find that their evolution is far from over.